Preamp for ST70

knowledge base for the classic Dynaco ST70

Preamp for ST70

Postby kt88pppamp » Wed Oct 08, 2008 7:32 pm

I am going to purchase a stereo 70 from Triode and use Blackburn Audio's 6SN7 driver board. The creator of the board in an e-mail told me, if I am using a 1 volt load I need a gain of 2.6 from a preamp.

In tube cad I am having trouble getting a gain that low. If I use higher gain than that I will probably have distortion and noise problems.
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12B4A s

Postby EWBrown » Thu Oct 09, 2008 2:05 pm

2.6 is probably the minimum requiired gain, if you have more, just don't crank it up to "eleven" Yellow_Light_Colorz_PDT_03

Any linestage using low-mu triodes should work, a good choice is 12B4As, run them around 250V on the plate, 10 mA plate current, 12K, 5W plate resistor and 1500 ohms, bypassed (or not) with 100-470 uF cap. No bypass cap will offer lower gain and some nbuilt-in negative feedback.
Take the output off the plate through a 0.22 to 0.47 uF / 400V coupling cap, and don't forget a 100K to 470K resistor from output to signal ground to drain off any DC voltages from the output. The above values will set the 12B4A plate voltage at approx 1/2 the B+, or 125VDC. THe cathode voltage will be approx 15VDC, which may seem high as compared to 12AU7 or 6SN7, but is normal for this tube.

The 24 step "ladder" attenuators from Hong Kong or Taiwan are a good choice for volume control.

6SN7s 6CG7s or 12AU7s in VA/CF would work, though the gain would be higher, around 14 or so.

To "gild the lily", a 6JZ8 regulated B+ power supply, DC filaments and plate CCS using IXYS 10M45 current regulators would really flesh the 12B4A linestage out.

The design idea isn't mine, I got this from Gary Kaufman. Someday I'll actually build one Yellow_Light_Colorz_PDT_11

/ed B
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